Regional distance shear-coupled PL propagation within the northern Altiplano, central Andes

Properties of the shear-coupled P wavetrain (SPL) from regional earthquakes provide important information about the structure of the crust and upper m antle. We investigate broad-band seismic data from intermediate-depth earth quakes and develop a grid search technique using synthetic seismograms to s tudy the sensitivity of SPL and to model the crustal structure of the north ern Altiplano, central Andes. Waveforms from an earthquake that occurred on 1994 December 12 within the N azca slab beneath the Altiplano display a clear SPL wavetrain at the tempor ary stations deployed during the BANJO and SEDA experiments. We relocate th is event and determine the moment tensor by inverting the complete long-per iod waveforms. With these source parameters fixed, we perform sensitivity a nalyses using a reflectivity technique to compute synthetic seismograms at a distance of 313 km (BANJO station 2, SALI). We find that, at this distanc e, the long-period SPL wavetrain is sensitive to the following model parame ters, in order of decreasing sensitivity: crustal V-P/V-S, mantle V-P/V-S, average crustal velocity, crustal thickness, focal depth, distance (locatio n), crustal Q(alpha) and Q(beta), and mantle velocity. We develop a grid search method to investigate the four parameters of the c rust/upper mantle model to which the synthetic seismograms are most sensiti ve at SALI (crustal V-P/V-S, mantle V-P/V-S, average crustal velocity, crus tal thickness). Trade-offs exist among all four of the model parameters, re sulting in a range of acceptable crustal models that provide excellent fits between the data and synthetic seismograms in the passband of 15-100 s at a single station. However, by using data at a range of distances (150-450 k m) we find that the model that provides the best overall fit between the da ta and synthetic seismograms, and thus best approximates the average charac teristics of the crust and upper mantle structure of the northern Altiplano , is characterized by an average crustal velocity of 6.0 km s(-1), a crusta l Poisson's ratio (sigma(crust)) of 0.25 and a mantle Poisson's ratio (sigm a(mantle)) of 0.27. The resulting models confirm previous estimates of low Poisson's ratios, low average crustal velocity and thick crust in the Altip lano.